Asymmetric focusing microlens array fabricated using off-axis lithography

Abstract

This research aims to make a microlens with a given inclined angle and applied to the light signal processing system. With two circular pattern masks with different diameters, a circular photoresist column structure is firstly made with the photolithography, whose first layer would be got after the small sized circular mask is taken for exposure and development. It is finalized through hard bake and then sputtered with a layer of metal (Cu), and then remained by taking the lift-off method. In the second part, the large sized circular mask is used. And the large sized circular pattern is shifted toward one direction to cover the first layer of metal pattern for the second exposure by taking the counterpoint exposure method to obtain the photoresist column structure. During the process of thermal reflow, there is no change in surface morphology of the first layer of metal pattern since it has been fixed. However, in the second layer of photoresist which goes through the deviation counterpoint exposure, the photoresist column is converted to rubbery state when its temperature is increased to the glass transition temperature (Tg) during the thermal reflow. Then the asymmetric microlens structure can be formed by shifting the arc vertex of microlens toward one direction with the fact that the copper coating surface is superior to silicon substrate surface in hydrophobicity. In this research, a 55° asymmetric microlens array can be manufactured by properly controlling the copper pattern size and the offset of two centers with the above method.